There can be a lot of causes for lethargy. The most common one (and most overlooked) is a parasite problem. It can rob the body of nutrition, dehydrate the dog and cause death if left untreated.
A urinary blockage is also a likely culprit.
Intestinal obstructions or even a general virus can cause death if left untreated.
I wish that I could give you a more definitive idea but there are just too many to consider.
You are correct in that you don't normally see parvo in a dog this age.
The only way to find out for sure what the cause of death was is to see the vet for a necropsy (pet autopsy).
Let me know if you still have questions.
And I am very sorry for your loss.